Terrestrial Planets
The rocky inner worlds, Mercury, Venus, Earth, and Mars, and the geological and interior processes that distinguish them.
Definition
Terrestrial planets are the dense, predominantly rock-and-metal planets, comprising Mercury, Venus, Earth, and Mars, studied through their interior structure, surface geology, and long-term evolution.
Scope
This area covers the silicate-and-metal planets of the inner Solar System and their differentiated interiors, surface geology, volcanic and tectonic histories, impact records, and divergent climate and volatile evolution. It treats them comparatively, asking why planets that began with broadly similar materials developed into a hothouse, an active living world, a frozen desert, and an airless cratered globe. Methods include spacecraft remote sensing, in-situ and sample analysis, geophysics, and modelling of interior structure and thermal evolution.
Sub-topics
Core questions
- How do rocky planets differentiate into core, mantle, and crust, and how do they lose their internal heat?
- What controls whether a planet has plate tectonics, stagnant-lid convection, or volcanic resurfacing?
- Why did Venus, Earth, and Mars follow such different climate and volatile paths despite similar origins?
- What do impact craters and surface features record about a planet's age and history?
Key theories
- Planetary differentiation
- Rocky planets heated by accretion and radioactivity melt and segregate into a dense metallic core, a silicate mantle, and a crust, setting up the layered structure that drives their later geological and magnetic behavior.
- Mantle convection and thermal evolution
- Solid-state convection in the silicate mantle transports interior heat to the surface and governs volcanism, tectonics, and the cooling history of a terrestrial planet.
- Comparative climate divergence
- Small differences in solar distance, mass, and volatile inventory drove Venus, Earth, and Mars to dramatically different atmospheres and surface conditions through feedbacks like the runaway and reverse greenhouse.
Clinical relevance
The terrestrial planets are the natural laboratory for understanding Earth in context, including the conditions that make a planet habitable and the processes that shape any rocky world, in or beyond the Solar System.
History
Comparative study of the rocky planets began in earnest with the Mariner, Venera, and Viking missions of the 1960s and 1970s, which revealed Venus's crushing greenhouse, Mars's cratered and channelled surface, and Mercury's airless terrain. Decades of orbiters, landers, and rovers, together with geophysical modelling, have built a unified picture of how rocky planets differentiate, cool, and evolve.
Debates
- Why only Earth has plate tectonics today
- Why Earth sustains plate tectonics while Venus and Mars do not is unresolved, with water content, lithospheric strength, and thermal history all proposed as controlling factors.
Key figures
- Harold Urey
- David J. Stevenson
- Gerald Schubert
- H. Jay Melosh
Related topics
Seminal works
- melosh2011
- turcotteschubert2014
- stevenson1981
Frequently asked questions
- Which planets are the terrestrial planets?
- Mercury, Venus, Earth, and Mars, the four inner planets made mostly of rock and metal rather than gas and ice.
- Why is Venus so much hotter than Earth?
- Venus has a thick carbon-dioxide atmosphere that traps heat in a runaway greenhouse effect, making its surface hot enough to melt lead despite its similar size to Earth.